In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon a detector, which is an array of radiation detector cells. The intensity of the attenuated beam radiation received at the detector cell array is dependent upon the attenuation of the x-ray beam by the object. Each detector cell of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detector cells are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The CT system x-ray source typically includes an evacuated x-ray envelope containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by applying a high voltage across the anode and cathode. The x-rays diverge from the focal spot in a generally conical pattern.
In some known CT systems, the x-ray beam from the x-ray source is projected through a pre-patient collimating device, or collimator, that restricts the x-ray beam in the patient axis, or z-axis. More particularly, one known x-ray collimator has a substantially circular cross-sectional shape. A number of apertures extend through the collimator, and each aperture corresponds to a particular slice width. For example, a first aperture corresponds to a 10 mm slice width and a second aperture corresponds to a 7 mm slice width. If a scan is to be performed for a 10 mm slice, then the first aperture is aligned with the expected x-ray focal spot and restricts the beam projected from the focal spot to the 10 mm slice width.
With respect to the CT system detector array, one known array is formed by fifty-four (54) modules. Each module has sixteen (16) cells. A detector cell having a degraded z-axis profile can cause image artifacts that significantly reduce image quality.
To detect z-axis profile cell degradation, detector cell testing is frequently (e.g., every six months) performed at the CT system site. These tests are very tedious and operator intensive. Particularly, as part of such tests, field engineers acquire several sets of scans with flat and sloped phantoms, and images are reconstructed using such scans. The reconstructed images are then subjected to center spot and band artifact analysis. The analysis results are used to identify degraded detector cells that should be replaced.
Another approach, rather than such extensive testing, is to implement an artifact correction algorithm for removing, or at least reducing the effects of, detector cell z-axis profile degradation. Such artifact correction algorithms do not, however, correct the root cause of the problem, i.e., detector cell degradation, and implementation of such a correction algorithm may be very expensive.
It would be desirable, of course, to determine z-axis profile detector cell degradation without requiring tedious and operator intensive testing. It also would be desirable to make such determinations in the field.